SAW technology has found a number of applications in the electronics and RF art. Due to the fact that SAW wavelengths are typically 10.sup.5 times shorter than that of electromagnetic waves, SAW technology has found particular applications where miniaturization is important or desirable. One such application is the use of SAW filters in radio telephones where the typically small size and weight of SAW filters is highly advantageous over conventional technologies, such as ceramic filters, dielectric filters, and filters using magnetostatic principles. It is a requirement of such filters that they have low-loss, typically insertion losses of 1.about.3dB.
A typical example of a conventional SAW filter is a transversal SAW filter in which SAW energy is transferred between two spaced apart interdigital transducers (IDTs). The IDTs each comprise two sets of electrode fingers which are formed on the surface of a piezoelectric substrate. The fingers in each set are typically all electrically connected together and are interleaved (interdigitated) with the electrode fingers of the other set. In the simplest form of IDT, the spacing between adjacent fingers of a set is one acoustic wavelength i.e., one per period. However, it is possible to have more than one finger per acoustic wavelength (period), and a corresponding number on the other set. In a transversal SAW filter, electromagnetic energy is converted into SAW energy by coupling the electrostatic field pattern of an input IDT to a SAW by the piezoelectric effect. A problem with SAW filters is that their maximum input power is limited due to the mechanical vibration caused by large amplitude SAWs degrading the IDT electrode fingers resulting in lower performance from the filter. Additionally, conventional filters have relatively high losses, typically greater than 10 dB for transversal type SAW filters. Similar problems occur with SAW resonator type filters.
A known filter which addresses the problem of limited maximum input power is disclosed in an article entitled "Miniature SAW Antenna Duplexer for 800 MHz Portable Telephone Used in Cellular Radio Systems", IEEE MTT vol. 36 No 6 June 1988. The known filter utilizes electrically cascade-connected SAW IDTs of a type known as SAW resonators and conventional capacitors connected in a ladder-type scheme. In this scheme, the SAW resonators are substantially acoustically independent of each other and are conceptually modelled and used as electrical impedance elements. Modelling and using a SAW resonator as an impedance element is possible because a SAW element such as a SAW resonator has an electrical impedance which is, in part, dependent on the electro-acoustic interaction of the electrode fingers of the SAW resonator with the mechanical vibration of the SAW. Near to the center frequency of the SAW elements (i.e., the frequency at which the separation of adjacent fingers is .sup..lambda./ 2) it has a maximum electrical admittance and a minimum electrical admittance. These are respectively the electrical resonant and anti-resonant frequencies of the SAW element. When large changes in electrical impedance are desired, the electro-acoustic interaction must be high. Thus, SAW elements with a large number of electrode finger pairs are used. Conventional SAW resonators having reflectors at both ends of a transducer and a large number of electrode pairs can be used, or alternatively transducers having just a large number of finger pairs can be used. Since the SAW resonators in the known filter are being utilised primarily as lumped impedances, it is convenient to term them SAW impedance elements. The term SAW impedance elements will hereinafter be used when referring to any SAW element (IDT, SAW resonator or otherwise) which is being used in at least part for its electrical impedance properties. In the foregoing the individual SAW resonators can be modelled as lumped impedance elements connected in series, and a conventional capacitance (static capacitance C.sub.STot) connected in parallel between ground and a port of a SAW resonator. The static capacitance is due to the capacitance between electrodes of the SAW resonators, between electrodes of the SAW resonators and ground, and the resonator to resonator coupling pattern and ground.
The known filter can operate at high powers because in the passband of the filter, i.e., in the region of the SAW resonator center frequency, most of the signal energy is transferred electrically through the resonators. Thus, there is very little degradation of the electrode fingers. Although the above described filter addresses the problem of limited maximum input power, the out of band insertion losses are typically 20.about.25 dB which are not sufficient for many applications. Such low insertion losses are due to the static capacitance transmitting a significant proportion of energy in the out of band frequecy range through the filter, since out of band the filter acts as a capacitative ladder.